1. Field of the Invention
The present invention pertains to data transmission systems and more particularly to transmission systems that are used between surface data acquisition systems and downhole data gathering systems that compensate for standing wave nodes.
2. Related Prior Art
Prior art transmission systems between surface data acquisition systems and downhole data gathering equipment are fairly common in the oil industry. Most transmission systems are "hard wired" or require actual physical connections between the surface and the downhole equipment. Examples of relevant prior art systems are as described below.
U.S. Pat. No. 2,505,144, "Signaling System for Use in Mine Shafts", (Rutherford), relates to a system used to provide an signaling system for use in mine shafts as a means of communication between the hoist operator on the cage, or skip, during the movement of the cage. This system uses the hoisting cable as an autotransformer and is operative irrespective of its position, or depth, in the shaft, or conditions which may cause variations in the capacitance between the cage walls of the shaft.
U.S. Pat. No. 4,136,327, "Well Survey System Modulation Technique", (Flanders, et al.), relates to a communication system that uses a single pair of conductors in which a drill cable in a borehole is used not only to transmit power into the hole, and to communicate sensor data back up to the surface. A frequency for power and communication of 1 kHz is employed. However, this is frequency modulated by deviation of about five percent to transmit commands down hole. A different non-interfering phase modulation is used to send data back to the surface by causing a susceptance (preferably capacitive) to be connected across the supply cable to signify a binary pulse. The alternation in phase between current into the cable and potential across it is observed at the surface and the pulses so detected are processed by conventional data processing techniques. A coding method is used for sending commands into the hole. Each of a series of possible commands is assigned a binary value so that, for example, if commands two and four are to be sent, a total of two plus eight pulses (all of equal weight) forming a command word are sent down hole where they are counted by a binary counter. When the total number has been counted, outputs two and four of the counter will have signals on them which constitute the transmitted orders.
U.S. Pat. No. 4,017,845, "Circuitry for Simultaneous Transmission of Signals and Power", (Kilian, et al.), relates to a transmission line having a pair of wires enclosed in a shield which is used to simultaneously transmit high frequency signals and low frequency power between distant locations. At the sending end of the line a high frequency source is connected between the first and second wires by circuitry which prevents the low frequency power from being coupled into the high frequency source. A low frequency power supply is connected between the shield and both of the wires by circuitry which prevents the high frequency signals from being coupled into the low frequency supply. At the receiving end of the transmission line circuitry is provided which separates the high frequency signals from the low frequency power.
U.S. Pat. No. 4,156,869, "Conducting Cable", (Schukantz), relates to a cable for conveying information signals of selected frequencies and for simultaneously conveying a selected amount of electric power, in which a central conducting means is surrounded by a dielectric material. An outer conducting means is positioned around the dielectric, and cooperates with the central conducting means to provide a first path, through which the information signals are conveyed. One of the conducting means provides a second path, through which the electric power is conveyed.
U.S. Pat. No. 3,916,685, "Well Logging System and Method Using an Armored Coaxial Cable and Compensation Circuit", (Paap, et al.), relates to a well logging system which includes a transmitter in a borehole, having a sensor such as a radiation detector, a condition relating to the earth's formation traversed by the borehole. The transmitter provides data pulses which correspond in number and peak amplitude to the sensed condition. The transmitter also includes a reference pulse source and means for combining the reference pulses and the data pulses. The combined pulses are conducted to a receiver at the surface by an inner conductor of an armored coaxial cable. The shield of the armored coaxial cable is insulated from the inner conductor and from an outer cable armor which surrounds it. The shield provides a return path for the combined pulses thereby reducing the skin effect encountered when using the outer armor as a return path. High voltage for energizing the sensor is also conducted to the sensor by the inner conductor of the armored coaxial cable from a direct current power supply on the source. Low voltage is conducted by the shield of the armored coaxial cable from the power supply to the sensor, to the reference pulse source and to the combining means, while the outer armor of the armored coaxial cable provides a common ground connection between the transmitter, the power supply, and the receiver. The receiver includes a compensating circuit which monitors the reference pulses and adjusts the data pulses in accordance with the monitored reference pulses to compensate for deterioration of the data pulses during their transmission. The receiver also includes recording means which provides a record of the sensed nature of the earth formation in accordance with the compensated data pulses.